The proposed research examines the influence of photoperiod on the resetting (entrainment) response of the circadian pacemaker regulating rhythms of locomotor activity, pineal melatonin, and body temperature in mammals. Four groups of experiments are unified by the hypothesis that the circadian phase-shift response is determined by the phase relationship among the evening and morning components of a "complex" circadian pacemaker. First, circadian phase response curves (PRCs) will be measured following entrainment to long versus short daily photoperiods. The kinetics of advance and delay phase shifts will be measured and compared. comparison of responses among male and female Syrian hamsters and among Siberian hamsters of two distinct phenotypes will provide a basis for understanding species, sex and phenotypic differences in complex pacemaker properties. Second, the influences of pacemaker amplitude and retinal photosensitivity on phase-shift magnitude will be explored. third, this project will explore whether pineal melatonin or gonadal hormones feed back on the circadian pacemaker to affect the coupling of morning and evening oscillator components. Fourth, the project will explore how the intensity of light in the daily photoperiod alters pacemaker responses to photic stimuli. This work will lead to a greater understanding of the mechanisms by which light, melatonin, and reproductive hormones regulate the mammalian circadian timing system. The results will be relevant to issues of human health and productivity, including on-going research on jet-lag, shift- work and the chronobiology of mental-illness.